[out-of-ideas]

im sure it will be deleted/modified/adjusted/enhanced; though #1 is subjective, "unless it is essential" -> and right there, it is essential for google to do lookups of phone numbers to correct account ownership information based on another-companies paid services. "whenever possible try to provide" ... nope, it is never possible; phone validation or bust.

edit: i gotta change my first guess that it will be modified soon. google does not set the goal to not be evil, so they'll likely just leave the repo as-is, unattended.

quote: 1. An individual has a phone number

Some people do not own phones, or do not wish to provide you with their telephone number when asked. Do not require a user to provide a phone number unless it is essential, and whenever possible try to provide a fallback to accommodate these users.